test(e2e): golden-master Playwright net for the Platform refactor

Locks the five flows the refactor must not break (feed, channel, player,
settings, nav) so later sprints can prove they changed nothing. Adds a thin
data-testid layer and a dedicated seeded non-demo account; the suite runs on
the baked :8080 stack (authoritative) or :5173. No product-behaviour change.

frontend/package.json

@@ -22,6 +22,8 @@
     "react-i18next": "^14.1.2"
   },
   "devDependencies": {
+    "@playwright/test": "^1.61.1",
+    "@types/node": "^26.1.1",
     "@types/react": "^18.3.3",
     "@types/react-dom": "^18.3.0",
     "@vitejs/plugin-react": "^4.3.1",